Magento 1.7 has new feature to define the currency symbol from the backoffice.
To set the currency symbol for each currency,
goto System => Manage Currency =>Symbols
Then enter the symbol for each currency and save.
The position of the symbol ie. whether it appears before or after the price amount is defined according to the locale.
To change the position of the symbol for each locale
(eg. for french language) open up the file lib/Zend/Locale/Data/fr.xml in your favourite text editor
where fr is the language for which you want to change the position of the symbol.